Separation of methane by concentration of a gaseous mixture containing methane and carbon dioxide, such as natural gas, off-gas of oil field gas, blast furnace gas, burning furnace gas, etc., by means of a permeable membrane has been generally conducted at high temperature above 50.degree. C. These gaseous mixtures mostly contain acidic gases, such as sulfur dioxide, hydrogen sulfide, etc., in addition to carbon dioxide. Therefore, the membrane to be used for separation of methane from such a gaseous mixture is required not only to have selective permeability to carbon dioxide but also to have heat resistance, acid resistance, and hydrocarbon resistance as well as mechanical strength.
Conventional membranes for separation of methane include composite membranes mainly comprising cellulose resins, e.g., cellulose acetate and ethyl cellulose. However, these membranes comprising cellulose resins are inferior in heat resistance and hydrocarbon resistance and it has been difficult, therefore, to stably separate methane by the use of these conventional membranes.